The problem of container deformation in response to pressure differences existing between the inside of a closed container and the ambient pressure is well known in the packaging industry. Such container deformation may be non-recoverable for certain container materials, such as some rigid or semi-rigid structures made of plastics or metals. Thin-walled, flexible or partially flexible containers can be particularly sensitive to the problem.
While not wishing to be bound to any particular theory, there are a number of possible factors which may lead to the existence of the pressure differences between the interior and the exterior of the container mentioned above. The contents of the container may, for example, be chemically unstable or may be sensitive to certain contaminants such as might occur in a reaction between the gases which may exist in the head space of the container and the contents of the container, or alternatively, in certain specific circumstances, where the contents of the container may react with the container material itself. Any chemical reactions involving the contents may lead to either production of gases, and hence to overpressure in the container, or to the absorption of any head space gases thereby causing under pressure in the container. In addition, the solid contents may absorb moisture, such as created by condensation due to temperature differentials and become soggy or saturated.
Pressure differences between the pressure inside the container and the ambient atmospheric pressure may also occur when the temperature during the filling and sealing of the container is significantly different from external temperature during shipment, transportation and storage. Another possibility of a pressure difference may be caused by a different ambient pressure at the filling of the container from another ambient pressure at a different geographical location.
The prior art has proposed several solutions using valve systems which avoid pressure differences between the interior and the exterior of the container. Proposed solutions also relate to various venting caps which allow pressure generated inside the container to be released by escape of gas. U.S. Pat. No. 4,136,796 and EP 0 752 376 disclose self venting closures having a gas-permeable membrane covering an orifice to the exterior atmosphere. These membranes are made of a material which is impermeable to liquids, but permeable to gases. Therefore, these containers may have apertures to release gas to the exterior without losing their leak-tightness. U.S. Pat. No. 5,988,426 and EP 337677 disclose a vented lid that relies on a hydrophobic material to allow passage of air through the vent hole and prevent the passage of liquids through the vent hole. Another example U.S. Pat. No. 6,886,579 relies on a ball bearing mechanism to seal the vent and prevent spillage of liquid contents. Additionally, GB 1 146 972 discloses a venting cap to be fitted onto the mouth of a container. It allows the passage of gases while preventing passage of liquids through the venting membrane. This is achieved by choosing the size of the pores in the membrane.
The use of membranes in these applications can add a considerable expense to the venting system. Tests have shown that when containers are heated to sufficient temperature to cause internal pressures to develop, leakage through the membrane occurs. In the case of mechanical closures, these devices can also add complexity and cost to the vent system and can suffer from malfunction and breakage of the mechanical components. Therefore the need exists for a container for a flowable product such as liquid or particulate, or a cap for such a container, which allows venting of the container while preventing the leakage of the flowable contents from the container even under conditions where internal pressures exist.